Steam corrosion-resistant iron-chromium-aluminum-yttrium alloys and process for making same



United States Patent STEAM CORROSION-RESISTANT IRON-CHROMI-UM-ALUMlNUM-YTTRIUM ALLOYS AND PROC- ESS FOR MAKING SAME Carl S.Wnkusick, Cincinnati, Ohio, assignor to the United States of America asrepresented by the United States Atomic Energy Commission No Drawing.Filed Nov. 29, 1965, Ser. No. 510,476

3 Claims. (Cl. 29-182) The invention described herein was made in thecourse of, or under a contract with the US. Atomic Enengy Commission.

The invention relates to a selected class of improved steam corrosionand embrittlement resistant alloys of iron, chromium, aluminum, andyttrium.

Alloys of iron, chromium, aluminum, and yttrium were originallydeveloped for their oxidation resistance in air at temperatures over2000 F. As described in US. Patent 3,027,252, issued March 27, 1962, toJames A. 'McGurty and John F. Collins, the useful range of compositionsfor these alloys was as follows: 20.0 to 95.0 weight percent chromium,0.5 to 4.0 weight percent alminum, 0.5 to 3.0 weight percent yttrium,and the balance iron.

These alloys have also been found highly resistant to oxidation andcorrosion by superheated steam and thus potentially useful in solving acritical problem in nuclear reactor technology. One of the mostpromising approaches for obtaining increased efTiciency in thegeneration of power by nuclear reactors is the use of a superheatedsteam system wherein superheating is effected by direct passage of steamthrough the reactor core. Development of this type reactor has beenhampered by the lack of suitable fuel-element cladding and structuralmaterials.

While satisfactory with regard to oxidation and corrosion bysuper-heated steam, the mechanical properties of the above-describedironchromium-aluminum-yttriu-m system became adversely affected byprolonged exposure to superheated steam. Specifically, these alloysbecome severely hardened and embrittled within several hours attemperatures of about 340 C. to 540 C. and prolonged holding attemperatures from 540 C. to 705 C. may also result in embrittlement.Typical superheated steam nuclear reactor systems require an operatingsteam temperature of about 480 C. to 565 C. and a fuel elementtemperature up to about 675 C. The high probability of embrittlement andmechanical failure thus precludes use of the 20-95 iron-chromium, 0.54.0aluminum, 0.5- 3.0 yttrium alloys system for superheated steamenvironments.

Subsequently, I found that embrittlement in these alloys is reduced bymodifying the iron-chromiumalu-minum-yttrium system to a compositionconsisting of 0-20 weight percent chromium, 0.5l2.0 weight percentaluminum, 0.1-3.0 weight percent yttrium, and the balance iron. Thisaforementioned low chromium alloy system is described and claimed in mycopending application Ser. No. 357,845, assigned to the Atomic EnergyCom-mission. While this modified alloy composition amounts to asignificant impnovement in terms of lowering embrittlement, it has beenshown that the lowered chromium content results in excessive rate ofcorrosion by superheated steam environmerits, despite the presence ofaluminum and yttrium. Further it has also been determined that yttriumis detrimental to the notch impact properties of theiron-chromium-aluminum-yttrium system. The presence of yttrium even insmall amounts (i.e., 0.1-0.5 weight percent) increases theductile-tobrittle transition temperature to 100 C.200 C. as compared to25 C. for .a yttriumfree ironchromium-aluminum alloy. The structure ofthe alloy produced by the usual fabrication methods, such as melting,casting, and extrusion consists of an iron chromium-aluminum matrixcontaining a course dispersion of large particles of a brittleyttrium-iron (YFe compound phase. Impact data indicate that thesebrittle yttrium-iron particles act as internal notches and thus controlor shift the ductile-tobrittle transition temperature.

Where the iron-chromium-aluminum-yttrium system is to be used as a .fuelcladding, it must be drawn to a finished tubing above about 300 C. whichis above the ductile-to-brittle transition temperature. It would be moreeconomical to draw tubing at lower temperature; for ex ample, at roomtemperature. Where the iron-chromiumaluminum-yttrium system is to beused in a superheated steam environment, the steam corrosion rate mustbe reduced to an acceptably low rate, i.e., to less than a weight gainof 0.5 mg./cm. for at least 1000 hours. It is, therefore, an object ofthis invention to provide an alloy which has an acceptable level ofsteam corrosion resistance and which at the same time can be fabricatedinto tubing at low temperatures.

Among the specific objects of this invention are (1) to provide a steamcorrosion resistant iron-chromium-aluminum-yttrium alloy system; (2) toprovide an ironchromium-aluminurn-yttrium alloy system with enhancedhigh temperature strength; (3) to provide a method of increasing thestrength and impact resistance of the ironchromium-alummum-yttriumsystem; (4) to reduce the ductile-to-brittle transition temperature ofthe iron-chromium-aluminum-yttrium system; and finally (5) to satisfyany one or combination of the aforementioned objects.

Where the objective is to increase steam corrosion resistance ofso-called low chromium iron-chromium-aluiminum-yttrium alloys, i.e.,those containing 20% by weight chromium or less, my invention comprisespre-oxidizing an alloy consisting of 0-20 weight percent chromium, 0.5-12.0 weight percent aluminum, and 0.0l3.0 weight percent yttrium in anoxygen-containing atmosphere such as air.

Where the objective is to increase the strength, impact resistance, andis to lower the ductile-to brittle transition temperature of anyironchromium-aluminum-yttriurn alloy, the invention consists infabricating said alloy in such a manner as to effect dispersion of aninsoluble yttriumiron compound phase in an iron-chromium-aluminum matrixsuch. that the particles are less than about 1 micron in diameter. Inthe specific embodiment disclosed herein, this is accomplished byatomizing a molten mixture of a given iron-chromiumaluminurm-yttriumcomposition and then fabricating the resultant alloy powder by acombination of hot extrusion and hot rolling or swaging, followed bycold rolling to a desired finished size.

The iron 25 chromium-4 aluminum-1 yttrium alloy has been found toexhibit excellent resistance to steam corrosion over a wide range oftemperatures. However, as previously mentioned, the alloy is subjectedto embrittlement especially at temperatures below about 540 C. Thisembrittlement is caused by precipitation of a chromium-rich ferritephase. In order to reduce or eliminate the problem, the chromium contentmust be reduced. But when the chromium content is reduced to below about20% by weight, it causes a significant lowering of the resistance of thealloy to steam corrosion at temperatures in the range 550 C.-730 C.Increasing the aluminum content of the alloy is effective in improvingthe steam corrosion resistance but this causes a reduction in ductility.Instead of altering the composition of the low chromium (i.e., 20%chromium) alloy, this invention provides for a pre-oxidation in air atelevated temperatures to efiect a significant improvement in thesubsequent steam corrosion resistance. Theiron-chromium-aluminum-yttrium alloys containing 15% or less chromium,in particular, exhibit poor corrosion resistance in the temperaturerange 550 C.730 C. In the pre-oxidized state, however, the corrosionresistance is comparable to the high chromium alloys, i.e., alloyscontaining more than 20% chromium. The enhanced steam corrosionresistance phenomenon is illustrated in the following example.

Example I The remarkable increase in steam corrosion resistance isbrought about by the surface pre-oxidation of the low chromium alloy toproduce a continuous protective aluminum oxide coating and isgraphically illustrated in Table I below which is a summary of weightgain data for iron-chromium-aluminum-yttrium alloys which werepre-oxidazed in an air atmosphere at temperatures in the range 980C.1250 C. for the period of time indicated in the table, and exposed tosteam at temperatures in the range 550 C.730 C. for 4000 hours attemperature. Optimum pre-oxidation times and temperatures will depend onthe degree of protection required. Temperatures from 800 C. to over 1400C. will be effective because of the formation of protective aluminumoxides by selective oxidation of aluminum from the alloy. Attemperatures below 800 C., the oxides formed are not as protective asthose formed at higher temperatures. The preferred range is 980 to 1250C. requiring times from 1 hour to over 100 hours. At temperatures aboveabout 1250 C. it is necessary to carefully support the metal part toprevent distortion. Also incipient melting occurs as temperatures above1350 C. which is damaging to ductility.

TABLE I Preoxidation Treatment Steam Corrosion Testing in Air (4,000hours) Alloy Tempera- Time, hr. Tempera- Weight gain, ture, C. ture, 0.mgJcrn.

2541 None 730 0. 11 980 125 730 0. 00 1, 100 125 730 0. 11 1, 260 67 7300. 08 1541 None 730 0. 52 980 125 730 O. 02 1, 100 125 730 0. 03 1, 26067 730 0. 03 0501 None 550 0. 79 980 125 550 0. 06 1, 100 125 550 0. 1,260 67 550 0. 02 0561 None 730 0. 25 980 125 730 0. 05 1,100 125 7300.08 1, 260 07 7 30 0. 00 1041 None 550 5. 90 980 125 550 0. 04 1,100125 550 0. 00 1, 260 67 550 0. 15 1041 None 730 4. 48 980 125 730 0. 091,100 125 730 0.14 1, 260 07 730 0. 16

11 Alloy compositions are in weight percent:

2541=Fe-25 Cr4 A11 Y 1541=Fe-15 Cr4 Al1 Y 0561=Fe-15 Gr6 A11 Y 1041=Fe-(Jr-4 A11 Y It will be noted that the weight gain in air for all alloystested was relatively the same, indicating that the air oxidationresistance Was rather independent of the chromium content. On the otherhand, those low chromium alloys which were not pro-oxidized exhibited anextremely high weight gain when exposed to steam atmospheres. Note inparticular to 10 weight percent alloys which exhibited virtuallydisastrous weight gain during the 4000 hours steam test. In markedcontrast, a pre-oxidation reduced the steam corrosion resistance of thelow chromium alloys to the same level as the high, i.e. 20 weightpercent chromium-c n ai ing alloys.

As previously noted, embrittlement caused by precipitation of achromium-rich ferrite phase can be reduced or eliminated by lowering thechromium content, generally to 20% by weight or less. The addition ofyttrium to the high or low iron-chromium-aluminum system imparts asignificant measure of oxidation resistance to the resultant alloy, butit does so only at the expense of low temperature (room temperature toabout 200 C.) notch impact properties.

It has been theorized that if the yttrium could be forced into solutionor if the yttrium-iron, YFe precipitated particles were sufficientlysmall and uniformly dispersed in the iron-aluminum-chrominum matrix, thestrength as well as the impact propertie of the resultantiron-aluminum-chromium-yttrium alloy could be improved.

The following example illustrates the manner in which the theory wastested and confirmed.

Example II A mixture of iron, chromium, aluminum, and yttrium containing3 weight percent yttrium was melted and atomized in an argon atmosphereto form a fine powder of the alloy corresponding to the samecomposition. The powder particle size was about 100 microns in diameter.The atomized powder was cold pressed in a mild steel casing to 10,000p.s.i. The casing was sealed and extruded at a reduction ratio ofapproximately 14:1 at 1000 C.

' This was then followed by hot rolling at 1000 C. and

finally by cold rolling to produce a finished sheet specimen. Stressrupture tests were then conducted on the sheet specimen at temperaturesbetween 650 C.-800 C. after the material was given a preliminary anneal.For purposes of comparison, a 5 chromium, 6 aluminum, 1 yttrium, balanceiron alloy was made by casting, extruding, and rolling to a finishedsize and tested in the same manner. The results are summarized in TableII below.

TABLE II Hr. Rupture Stress at Indicated Temp., kg. em. Alloy 650 C. I700 C. l 750 C. I 800 0.

0561 Fe-S Cr6 Al-1 Y... 387 253 183 134 0563-.." Fe5 Cr-G A13 Y 775 492330 211 The results shown above indicate a significant strengthimprovement of the atomized 3 weight percent yttrium alloy vs. the 1percent yttrium which had been cast and extruded.

Impact specimens were then machined from these alloys and it was foundthat the atomized specimen containing 3 weight percent yttrium had aductile-to-brittle transition temperature of about 50 C. While the castand extruded 1 weight percent yttrium-containing alloy had aductile-to-brittle transition temperature of about C. Thus, the decreasein the transition temperature for the 3 Weight percent alloy is over 100C. in spite of the higher yttrium content. This result indicates thatthe embrittling effect of yttrium in the basic ironchromium-aluminumalloy is not so much a function of the amount of yttrium added, but ismore closely related to the physical form it takes in the resultingmetallurgical structure. Thus, the oxidation and steam corrosionresistance of a 1% yttrium-containing alloy is comparable to a 3%yttrium-containing alloy. However, where both alloys are formed bymelting, casting, or extrusion, the 3% yttrium alloy will have reducednotch impact properties and will be diflicult to fabricate into sheetform for example. On the other hand, when a 3% yttrium-containing alloyis prepared in such a way as to increase the solubility of yttrium inthe matrix phase or alternatively to form a fine dispersion of theyttrium in the ironchromium-aluminum matrix, the adverse embrittlingeffects Will be considerably ameliorated. Thus, with this occur withincreasing yttrium "concentration's.

elaboration of the part played by yttrium addition to a referenceiron-chromiumal-uminum system, I am now able to include a higher levelof yttrium to obtain simultaneously increased strength and impactresistance, Le, a lower ductile-to-brittle transition temperature.

Thus, the iron-chromiumaluminum-yttrium alloys derived, for example,from any process which either solubilizes the yttrium or allowsformation of :a finely divided dispersion of a yttrium-iron phase in aniron-chromiumaluminum matrix permits drawing of tubing at temperaturesbelow 300 C. The atomization technique is particularly advantageoussince it allows incorporation of quite small amounts in the range 0.01to 3 weight percent yttrium to a reference Fe-Cr-Al alloy to obtainoxidation as Well as steam corrosion resistance while reducing orameliorating the notch or embrittlement effects which In the atomizationtechnique, the yttrium is either retained in solution or precipitated asan extremely fine and fairly evenly distributed dispersion ofyttrium-iron particles. In general, if the particles are large, forexample 5 to 30 microns in diameter then the notch effect Will operateto increase the ductile-to-brittle transition temperature (as comparedto an ironchrominum-aluminum alloy without yttrium) to highertemperatures.

Methods other than atomization may be used to produce the desireddispersion. These include milling or other fragmentation processes. Itis not possible to heat treat the alloy to obtain the dispersion becauseof the very low solubility of yttrium in the solid base metal.Atomization is the preferred process because yttrium is soluble in theliquid metal. Also, the rapid quenching feature involved in theatomizing process prevents large yttriumiron compound particles fromforming.

Having thus described my invention, I claim:

1. A dispersion-strengthened alloy of iron-chromiumaluminum and yttriumconsisting of an iron-chromiumaluminum matrix and a finely dispersedphase iron and yttrium, the particles of said dispersed phase having amaximum diameter no greater than 5 microns.

2. A method of fabricating an alloy from an alloy system comprising 0-25chromium, .5-12 aluminum, 0.01-4 yttrium, and the balance iron whichcomprises melting an alloy of a composition selected from said systemand rapidly quenching the melted alloy in such a manner as to produce aprecipitated yttrium-containing phase containing a predominant amount ofparticles having a maximum diameter of less than 5 microns.

3. A method of producing a fine dispersion of a yttrium-containing phasein an alloy having a matrix containing iron, chromium, and aluminumwhich comprises atomizing a molten composition of iron-chromium-aluminumand from 0.01-4 weight percent yttrium to form an alloy powder whosemetallurgical structure consists of a dispersion of yttrium in aniron-chromium-aluminum matrix, the particles of said dispersed phasehaving a maximum diameter no greater than about 5 microns.

References Cited by the Examiner UNITED STATES PATENTS 2,061,370 11/1936Rohn -124 2,190,486 2/1940 Schafmeister 75--123 2,864,734 12/1958 Adamset a1 148-6.35 2,890,974 6/1959 Carrigan 148-6.35 2,909,808 10/ 1959Frehn.

3,027,252 3/1962 McGurty et al 75-124 3,194,658 7/1965 Storcheim 75224L. DEWAYNE RUTLEDGE, Primary Examiner.

r R. L. GRUDZIECKI, Assistant Examiner.

1. A DISPERSION-STRENGTHENED ALLOY OF IRON-CHROMIUMALUMINUM AND YTTRIUMCONSISTING OF AN IRON-CHROMIUMALUMINUM MATRIX AND A FINELY DISPERSEDPHASE IRON AND YTTRIUM, THE PARTICLES OF SAID DISPERSED PHASE HAVING AMAXIMUM DIAMETER NO GREATER THAN 5 MICRONS.
 2. A METHOD OF FABRICATINGAN ALLOY FROM AN ALLOY SYSTEM COMPRISING 0-25 CHROMIUM, .5-12 ALUMINUM,0.01-4 YTTRIUM, AND THE BALANCE IRON WHICH COMPRISES MELTING AN ALLOY OFA COMPOSITION SELECTED FROM SAID SYSTEM AND RAPIDLY QUENCHING THE MELTEDALLOY IN SUCH A MANNER AS TO PRODUCE A SPECIPITATED YTTRIUM-CONTAININGPHASE CONTAINING A PREDETERMINANT AMOUNT OF PARTICLES HAVING A MIXIMUMDIAMETER OF LESS THAN 5 MICRONS.